LibWeb: Correctly initialize Storage objects on the Document

Instead of storing all storage objects in static memory, we now
follow the the spec by lazily creating a unique Storage object
on each document object.

Each Storage object now holds a 'proxy' to the underlying backing
storage. For now, this proxy is simply a reference to the backing
object. In the future, it will need to be some type of interface
object that stores on a SQLite database or similar.

Session storage is now correctly stored / tracked as part of the
TraversableNavigable object.

Local storage is still stored in a static map, but eventually this
should be factored into something that is stored at the user agent
level.

Libraries/LibWeb/StorageAPI/StorageShed.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2024-2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/HTML/Scripting/Environments.h>
+#include <LibWeb/StorageAPI/StorageShed.h>
+
+namespace Web::StorageAPI {
+
+// https://storage.spec.whatwg.org/#obtain-a-storage-shelf
+Optional<StorageShelf&> StorageShed::obtain_a_storage_shelf(HTML::EnvironmentSettingsObject const& environment, StorageType type)
+{
+    // 1. Let key be the result of running obtain a storage key with environment.
+    auto key = obtain_a_storage_key(environment);
+
+    // 2. If key is failure, then return failure.
+    if (!key.has_value())
+        return {};
+
+    // 3. If shed[key] does not exist, then set shed[key] to the result of running create a storage shelf with type.
+    // 4. Return shed[key].
+    return m_data.ensure(key.value(), [type] {
+        return StorageShelf { type };
+    });
+}
+
+// https://storage.spec.whatwg.org/#user-agent-storage-shed
+StorageShed& user_agent_storage_shed()
+{
+    // A user agent holds a storage shed, which is a storage shed. A user agent’s storage shed holds all local storage data.
+    // FIXME: Storing this statically in memory is not the correct place or way of doing this!
+    static StorageShed storage_shed;
+    return storage_shed;
+}
+
+}